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32 pendicular line is the equator. For an observer at either pole would observe the sun on his horizon for the 24 hours of that date, in other words, the sun would not rise or set. The sun itself would be vertically over the equator, and its light just reaches either pole, which accounts for the sun being on the horizon for 24 hours. The same phenomena as above takes place about Sept. 21, at which time the declination of the sun is also zero. TAE Marine REVIEW INCLINATION OF THE EARTH'S AXIS. BIG 5: HOMELY DEFINITIONS. The geometrical word "plane" has been. used many times in the foregoing pages, such as the plane of the ecliptic, plane of the earth's orbit, etc. Men who have not had the advantages of mathematical in- struction sometimes fail to comprehend its meaning, and a reference to the dictionary leaves them as mystified as ever. In the Pole MI Plane of Ecliptic Sue ; qto = = = TiS Inclination of the earth's axis----The absolute direction of the earth's axis at all points of the orbit, is nearly the same. This direction makes an angle of about 6614° with the plane of the ecliptic--a plane passing through the earth's orbit and the sun's center. The axis, there- fore, leans about 2314° (90°--66%4°) out of a perpendicular to the plane of the ecliptic. | clination of the axis. CONTINUOUS DAY OR CONTINUOUS NIGHT. Beyond the tropics the sun, in the hemisphere in which it is vertical, makes the entire circuit of the heavens, without sinking below the horizon, for a period varying from 24 hours to six months; while in the opposite hemisphere there is a corresponding period of continuous night. . THE LENGTH OF THE DAY AND NIGHT. The following table gives the length of the longest day, excluding the time of twilight, and of the shortest night, in the different latitudes, with the difference of duration in hours and minutes, thus ex- -hibiting more clearly the above law. Latitude. Longest Shortest Differ- Day. Night. Equator 12.0hours 12.0hours 0.0 10° Mfrs ilesiat 1.4 20° 133s OMe 2.6 BleGopics ls.on GES aeas 3.0 Sue 140 " GMO) = 40 Bare Wise ee OSes 5.0 40° ESS ae Dies 6.0 45° 1LSy Saves to Ores 7.2 50° os: dere 8.6 Joe Wistar Oe te OG 60° TSE a Ose 134 Polar Circles 24.0 " OO 20) 6714° 1 month 6914° 2 months WA 3 months LO3% 4 months 84° 5 months North Pole 6 months This angle constitutes the in- EXPLANATION OF Fic. 5. cases we are dealing with, the plane is supposed to be a flat horizontal surface without thickness. Perhaps the nearest resemblance to such an ideal condition will be found in a sheet of tissue paper held perfectly horizontal. The perfectly smooth surface of the body of water rep- resents an ideal plane. Who has not seen the lake as smooth as glass, stretch- ing away in every direction as far as the eye can reach. This then is your plane, pure and _ simple. The smooth horizontal surface of a floor, looking- glass, etc., represents a plane. A plane must be a surface without curvature. A plane may be real or imaginary. The plane of the ecliptic and the plane of the equator, are, of course, imaginary. A plane is not necessarily horizontal. It may be a vertical plane, or an oblique plane. Now, what is meant by the plane of the ecliptic? The first thing to con- sider is, what is the ecliptic? The eclip- tic is the real yearly path of the earth around the sun as seen from the sun, or the apparent path of the sun in the sky. The ellipse marked orbit in Fig. 1 then is the same thing as the ecliptic. The plane of this ecliptic then must be the flat (horizontal) surface lying wholly within the confines of the ellipse or or- bit. Lay this magazine on a table and turn to Fig. 1. The plane of the ecliptic then is the flat surface of the sheet of paper lying within the orbit. This is the same thing as the plane passing through the earth's orbit. The plane of the ecliptic is also defined as that plane pass- ing through the center of the sun, which contains the orbit of the earth. The line, or plane, to reach from the sun's center to the orbit of the earth, must, theoreti- cally, pass half way through the earth to its center, if we consider the orbit to be only a line, hence half of the earth lies on either side of the line. If the path, or earth's wake, were considered to be the same width as the diameter of the earth itself, the plane of the ecliptic would have to pass all the way through the earth to contain the full width of the orbit. No matter how it is considered, the plane to pass from the sun's center to the orbit, must pass through the cen- ter of the earth, no matter where the earth may be in its orbit. This does not mean that the plane has to pass through the equator in order to reach the center cf the earth. This is true only twice in the year, during the equinoxes. At all other times the earth occupies such a po- sition' in its orbit that there is an angle between the plane of the ecliptic and the Se ldive sol thes equator Sometimes the plane of the ecliptic strikes be- low the equator, and sometimes above. Wherever the plane strikes the outside of the earth the angular distance from there to the plane of the equator, er just simply to the equator, is equal to the sun's declination. What is meant by the plane of the equator? A plane is an extended flat surface without thickness. Just imagine a sheet of cardboard of such dimensions that half of it would reach from the earth to the sun. Next, imagine a cir- cular hole cut in the center of the card- board just large enough to admit the earth. It wants to fit tight so that the earth cannot drop through the hole. Now, imagine the sheet of cardboard drawn over the earth and fitted so as to coincide with the equator all the way round. When this is done the sheet of paper represents the plane of the equa- tor and extends from it in all directions. Half of the earth will be above the plane and the other half below it. Now, imagine the same thing done with the sun. The sun's axis is not inclined like that of the earth, and even though it were, it would make no difference, since the heat and light are equal all over its surface. This statement is not strictly true, but since it has nothing to do with this case, it will make no difference. Now, imagine the sun as a ball, as you 'see it on the sea horizon, on a bright morning. Imagine another sheet of card- board stretched around the sun like that around the earth. This time the sheet of cardboard must be perfectly horizon- tal, and must encircle the sun at its mid- dle, one-half above, the other below. The sheet of cardboard represents the plane of the ecliptic, and if carried to the earth would ultimately _ strike its scenter, and 4f «the earth «in its orbit happened to be in that position where it was inclined from the plane of the ecliptic, or the plane from the sun, as it might be called, the plane of the ecliptic would strike the earth on one side or the other of the piane of the equator, or simply to one side or the other of the equator itself. A study of Fig. 5 ought to make this